Magnetic moment dipolar coupling

The relaxivity enhancement of water protons in the aqueous solutions of paramagnetic complexes arises from time fluctuation of the dipolar coupling between the electron magnetic moment of the metal ion and the nuclear magnetic moment of the solvent nuclei (13,14). The dipolar interaction... 

In principle, there are several contributions to nuclear Tm" however, the dipolar coupling term often dominates (10,22). The dipolai contribution depends on the reciprocal of the sixth power of the distance between the resonating nucleus and the relaxing electron, on the square of the magnetic moments associated with the unpaired electrons (ge2 B S(S+l)) and with the nucleus on the magnetic field as expressed by the Larmor... 

It was noted that for 78b and 79b, 7hf and 7cf are dependent on the molecular conformation, but this is not the case with 7nf. which is independent of the conformation. It is well known that the internuclear couplings are electron coupled interactions for which there are three possible mechanisms (1) the nuclear moments interact with the electronic currents produced by the orbiting electrons (2) there is a dipolar interaction between the nuclear and electronic magnetic moments (3) there is an interaction between the nuclear moments and the electronic spins in i-orbitals, the so-called Fermi contact term. ... 

D spectra are in principle possible for heteronuclei coupled by either dipolar or scalar interactions. However, the magnetic moments of heteronuclei are sizably smaller than that of the proton, and since cross relaxation depends on the square of the magnetic moment it appears that this is a serious limitation for the observation of NOESY or ROESY cross peaks. However, as already discussed, in scalar-coupled systems the relevant coherences build up with sin(nJ/jt). Since Jjj in directly bound 13C- H and l5N- H moieties is of the order of 102 Hz, as opposed to about 10 Hz between proton pairs, it is conceivable that scalar correlation experiments are successful. Heterocorrelated spectra have the advantage of allowing one to detect signals of protons attached to carbons or nitrogens when they are within a crowded envelope. 

In paramagnetic systems, there can be non-negligible contributions to the CSA of NH nuclei from their dipolar coupling with the time-averaged magnetic moment of the electron (see Section 3.6). As a consequence, it cannot be predicted a priori which will be the sharpest component for each NH peak in a heterocorrelated experiment, and TROSY is less useful, unless four different TROSY spectra are acquired by selecting a different component each time [27]. 

The Hamiltonian for the dipolar coupling between two magnetic moments associated with the spins I and J is given by an equation similar to Eq. (1.1)... 

The direct (dipolar) nuclear spin-spin coupling constant represents the classical through-space interaction of the magnetic moments of nuclei K and L. 

Owing to their proximity, these are coupled by their dipolar fields to produce a magnetic moment. This has been termed the Kuhn-Kirkwood mechanism, which has been extended to include degeneracy by Moffitt. 

This is a very important result. The first term in the last line of (4.13) represents the so-called Fermi contact interaction between the electron and nuclear spin magnetic moments, and the second term is the electron-nuclear dipolar coupling, analogous to the electron-electron dipolar coupling derived previously in (3.151). The Fermi contact interaction occurs only when the electron and nucleus occupy the same position in Euclidean space, as required by the Dirac delta function S(-i Rai). This seemingly... 

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